Non-stretching explosive cord

ABSTRACT

An explosive cord embodying high strength non-stretching means extending therealong and effective to prevent longitudinal stretching of the cord and thereby preventing malfunctioning and propagation failure due to interference with the continuity of the explosive charge. The stretch control filaments may be incorporated in the protective shroud for the cord or applied to the cord independently of the shroud. Additionally the invention cord may include a heat shield exteriorly of the enclosing tube for the explosive material and effective to retard the passage of heat through the latter under higher temperature ambient conditions as in deep earth bores.

May a, 1973 United States Patent Grayson [54] NON-STRETCHING EXPLOSIVE CORD Primar Examiner-Verlin R. Pende rass If. Y E [75] Inventor BobbyW Grayson Oildale Cal Attorney Sellers & Brace Assignee: Sellers and Brace, Pasadena, Calif.

Nov. 9, 1970 Appl. No.: 87,727

[57] ABSTRACT An explosive cord embodying high strength nonstretching means extending therealong and effective to [22] Filed:

prevent longitudinal stretching of the cord and Related 0.8. Application Data thereby preventing malfunctioning and propagation failure due to interference with the continuity of the explosive charge. The stretch control filaments may be i 07 W 02 7/] n 252 .l u o D. A 0 C 4 m 7 n 2 m 0 m N m r mm c u S n" 2 m" 2 M7 a P m we mm O mN mm t a u l m u .m 0 m C UI. i ll. 3 2 6 55 .l [.l.

incorporated in the protective shroud for the cord or applied to the cord independently of the shroud. Additionally the invention cord may include a heat shield [58] F eld extefimly of the enclosing tube for the explosive material and effective to retard the passage of heat through the latter under higher temperature ambient conditions as in deep earth bores.

References Cited UNITED STATES PATENTS 9 Claims, 13 Drawing Figures 1,539,018 Mott et al..;.......................... ;102/27 PATENTED MAY 8 I973 sum 1 or 3 INVENTOR y W 654 yap/v ATTOZA/ s49 PATENTED HAY 81973 SHEET 2 [1F 3 INV ENTOR 50512 n em 50M NON-STRETCHING EXPLOSIVE CORD This application has been divided from and is a continuation-in-part of my copending application for US. Letters Patent Ser. No. 27,214, filed Apr. 10, 1970, entitled Apparatus and Method of Conducting String Shot Operations, now US. Pat. No. 3,572,245.

This invention relates to explosives and more particularly to an improved explosive cord constructed to withstand high tensile stress without stretching and also incorporating heat shield means to retard harmful temexplosive operations conducted in earth bores, oil wells and deeper underground tunnels.

For example the string shot technique employed to perform servicing operations in oil wells and to improve the flow by clearing away material tending to clog the flow perforations is subject to many shortcomings and disadvantages avoided by the present invention. This technique utilizes an explosive cord supported along the side of a wire line and lowered to position the cord opposite a section of perforated casing to be cleared of debris. lt is not possible to determine the cause or seriousness of the clogged condition and more particularly how strong an explosive charge is required to clear the perforations yet avoid risk of damage to' the casing. It is costly and time consuming to carry out a string shot operation and this encourages the operators to use an excessively powerful charge to avoid having to perform several run in operations. Not infrequently, the extra strong charge seriously damages the casing.

Attempts to use explosive cords of low power or strength are uncertain and often result in failure or produce serious damage to the casing or parting of the wire line and loss of the weight required to pull the line into the well for quite different reasons. For example, in certain instances the cords malfunction or fail to explode or explode over only a small portion of their full length due to a failure of flame or shock propagation along the cord. For reasons that have not' been understood prior to the present invention, these lower strength lines would explode over only a short portion of their upper lengths. ln other instances either the wire line was severed or serious damage caused to the easing, or both, for reasons not understood or readily explained. It has been discovered that, in most instances, these accidents are produced by stretching of the cord due to a number of factors including the adverse affect on the cord of the very substantial sub-surface ambient temperatures customarily encountered at lower levels in oil wells andthe high drag forces acting on the cord as it is lowered through the mud and other fluids customarily present in a well. As a result of this stretching the distance between adjacent grains leads to propagation failure. lt can also result in excess portions of the stretched cord collecting in a loop adjacent an anchorage connection between the cord and the wire line. Upon detonation of the loop, an unusually powerful blast is produced in a localized area of the well which not infrequently cuts the wire line and releases the line weight but can cause serious damage to the casing itself.

It has been proposed heretofore to assemble a pair of explosive cords in parallel along the wire line and to explode them separately either by using a single or separate pairs of electrical leads. If the same pair is used, then both cords are set off in fast unregulatable sequence. The use of separate lead wires for each explosive cord enables the operator to vary the interval between detonations but is subject to the serious disadvantage of having to provide and protect separate leads for each charge. This complicates the equipment and servicing operation and involves the risk of severing one or more of the lead in wires and greatly increases the risk of malfunctions as well as the maintenance and handling costs.

Other problems associated with string shot operations as heretofore conducted involve the problem of equipping the wire line with centralizer devices to hold the line out of contact with the casing side wall and couplings during the run in and pull out cycle as well as to safeguard the explosive cord against damage during the lowering operation. These centralizers are required at frequent intervals along the wire line and their presence on the line interferes with reeling and storage of the line.

The foregoing and other serious disadvantages accompanying string shot operations as heretofore conducted are avoided by the present invention using the equipment and the improved techniques provided by this invention. A simple wire line is employed having a central conductor and a second conductor comprising the metallic sheath of the line which are utilized to supply the electrical energy required to detonate one'or a series of explosive cords in any desired time interval or simultaneously in one or more groups if so desired. Explosive cording incorporating means safeguarding against stretch and malfunctioning of the cord and retarding temperature rise of the explosive itself makes possible the use of lower explosive strength cord than heretofore possible and numerous other advantages.

An improved centralizer construction permits the centralizer to be installed and removed from the wire line expeditiously at the well head. After the main body of the centralizer has. been detached, the wire line can be compactly reeled on the'take up spool or storage reel. As soon as the centralizer is installed on the wire line one or more lengths of explosive cord can be secured to the periphery of its main body following which the wire line is lowered into the well until in posi tion to receive the next centralizer.

The selective detonation ofexplosive cords via a single pair of conductors is made possible by unique electrical discriminator means connected between one of the conductive paths and the detonators for each cord. Typically, the discriminator utilizes semi-conductors arranged to pass electrical energy only to a particular cord orto a set of cords selected for detonation at a particular time. Greater versatility is achieved by utilizing the discriminator in combination with a reversing switch and means for controlling the applied potential.

Another feature is the provision of a blow out protector of simple unique design across the well head. This protector includes a cover having a non-conductive gasket which fits tightly about the wire line in the closed position of the protector and which opens for the full diameter of the casing whenever the wire line is being lowered or withdrawn.

It is therefore a principal object of this invention to provide an improved explosive cord suitable for use under conditions likely to cause elongation of the cord and resulting in mal-or non-functioning of the cord.

More specifically it is an object of the invention to provide an explosive cord constructed to resist stretching under tensile stress.

Another object of the invention is the provision of an explosive incorporating high strength flexible means capable of resisting elongation of the tube charged with explosive.

Another object of the invention is the provision of an explosive cord incorporating a heat shield effective to retard the passage of ambient heat to the explosive material per se and very substantially extending the operating temperature conditions for such cord.

Another object of the invention is the provision of an explosive cord having built-in stretch control means and temperatiire shield means interfering in no material degree with the flexibility and handling characteristics of the cord but very substantially increasing its tensile strength, heat flow retarding characteristics and its usefulness under higher ambient temperature conditions.

These and other more specific objects will appear upon reading the following specification and claims and upon considering in connection therewith the attached drawing to which they relate.

Referring now to the drawing in which a preferred embodiment of the invention is illustrated:

FIG. I is a vertical sectional view of a well having a perforated casing and showing an illustrative embodiment of the invention string shot apparatus in position for detonation ofits separate cords;

FIG. 2 is a top plan view on an enlarged scale of the blow out protector closed against the well head;

FIG. 3 is a cross sectional view taken along line 3-3 on FIG. 2;

FIG. 4 is a cross sectional view taken along line 44 on FIG. 3;

FIG. 5 is a diagrammatic elevational view at thewell head showing details of the means for installing the explosive along the wire line at a desired uniform tension;

FIG. 6 is a schematic view of an illustrative embodiment of the electrical firing circuit fora plurality of explosive cords;

FIG. 7 is an elevational view on an enlarged scale of means for securing the explosive cords to an adjacent portion ofthe wire line;

FIG. 8 is a cross sectional view taken along line 88 on FIG. 7;

FIG. 9 is a fragmentary enlarged view of the invention non-stretching heat-shielded explosive cord;

FIG. 10 is an elevational view of one preferred embodiment of the wire line centralizer;

FIG. 11 is a top plan view taken on FIG. 10;

FIG. 12 is a cross sectional view taken along line 12- I2 on FIG. 10; and

FIG. 13 is an elevational view of a second preferred form of the centralizer assembled to the wire line.

Referring initially and more particularly to FIG. 1, there is shown an oil well casing 10 having a multiplicity of flow perforations ll distributed along its side wall at the production flow level. It will be understood that these perforations may be distributed over many feet of the casing and in one or more areas of the casing. Secured to the top of the casing is a radial flange 12 to which a blow out protector mounting ring 13 is detachably secured, as by bolts 15. This ring has a central opening 16, corresponding to the I. D. of the casing so as not to restrict the entrance to the well. Opening 16 is provided with a cover 17 here shown as formed in a single part and equipped with a hinge 18 secured to mounting ring 13. However, it will be understood that cover 17 may be formed in two halves each having its own hinge connection with mounting ring 13 if so desired. Threaded studs 20 welded or otherwise secured to mounting ring 13 pass through openings in cover 17 and support thumb nuts 22 for clamping the cover firmly sealed closed prior to a string shot firing operation.

Suitably secured to cover 17 and mounting ring 13 is a two part or split sealing gasket 23,24 having its edge portions shaped to clamp snugly about the wire line 25 when cover 17 is closed. Each of the gasket halves 23,24 are-suitably and firmly anchored to the cover and to the mounting ring as in the manner indicated in FIGS. 2 and 4. These non-conductive sealing gaskets not only provide a fluid-tight seal about the wire line but isolate the line and its conductors electrically from the casing.

Wire line 25 includes a central conductor 26 separated from the helical sheath conductors 27 which also serve as the second conductive path employed as will be explained presently to provide one portion of the electrical circuit between the direct current power source 28 (FIG. 6) and one or more string shots. Typically wire line 25 passes over a guide pulley 30 overlying the well and thence to a storage reel, not shown, but of any suitable well known construction driven by reversible power means for handling the wire line.

Referring now more particularly to FIGS. 1, 10, 11 and 12, it will be understood that wire line 25 includes a weight 35 at its lower end of adequate size to facilitate the expeditious lowering of the wire line into the well through the thick fluids generally present in the well. Distributed along the lower portion of the wire line at suitable intervals, as IO to 25 feet, are a plurality of centralizers 40,40. As is best shown in FIGS. 10 to 12, each centralizer includes a mandrel 41 having a diameter similar to that of wire line 25 and provided at its opposite ends with yokes 42. These yokes are formed with aligned openings 43 each equipped with a pivot pin 44 suitable secured in place and extending through a loop 45 of the wire line or of a similar cable 46 interconnecting adjacent ones of the centralizer mandrels.

Loosely fitting over the mid-portions of mandrels 41 is the main body 48 of the centralizer. This main body is generally cylindrical in contour and has a long deep slot 49 opening through one side wall and sized to fit readily over mandrel 41. Main body 48 is slightly shorter than the distance between the adjacent ends of yokes 42, and is formed with a threaded bore seating a thumb screw 50 having a knurled head. Its inner end extends into slot 49 in position to engage the mandrel and hold the centralizer body firmly locked in assembled position in the manner made clear by FIG. l2.

Extending crosswise of the opposite ends of the cen tralizer bodies 48 are pairs of passages 51 loosely seating highly tempered wire loops 53. As herein shown, each centralizer is provided with a single pair of these wire loops shaped as best shown in FIG. and each including oppositely directed semi-cylindrical loops projecting radially from .the side wall of main body 48.

Since each of these loops lies at an angle to the portions located in passages 51, the loops are held against sliding movement lengthwise of these passages. The radial extent of each of the loops is such that they cooperate in holding the wire line loosely centralized in the well casing.

A second and simpler embodiment of the invention centralizer 40, illustrated in FIG. 13, may be mounted directly on wire line 25 at anysuitable spacing therealong. For this purpose the wire line is provided with collars 60 telescoped over one end of the wire line and then subjected to high pressure contraction forces adequate to squeeze the ductile collar material into interlocking gripping action with the wire sheath of the wire line. These collars pass readilyover the storage reel for the wire line.

The main body 48' of the centralizer is formed with slot 49' similar to the corresponding slot in FIGS. 10 to 12 and sized to fit readily over the wire line. Intermediate the opposite ends of this slot there is an enlarged U-shaped recess 61 shaped to fit loosely about and accommodate collar 60 in the manner made clear by FIG. 13. A thumb screw is carried in main body 7 centralizer detachably assembled to the wire line whereas collar cooperates with recess 61 to hold the centralizer against movement longitudinally of the wire line. The simplified centralizer avoids the need for the yoke-equipped mandrels as well as for separate lengths of cabling 46 interconnecting adjacent centralizers.

The assembly of the explosive cord through the wire line will now be described with the aid of FIGS. 1 and 5. The crew attaches weight 35 to the lowermost end of the wire line and inserts this weight into the top of the well. As shown in FIGS. 1 and 5, only a pair of explosive cords 65,66 are shown. In many operations a single pair suffices but in others it may be desirable to fit the wire line with a larger number of string shots. In this case, it is usually preferable to use explosive cord of considerable smaller size and explosive power than that heretofore employed. For example, it has been common practice heretofore not to use explosive cord smaller than 25 grain size in string shot operations. However, by this invention, it is feasible and advantageous to use cord of the smallest commercial size, namely four grain as well as various sizes intermediate four grain and 25 grain provided adequate precautions are taken against elongation of the cord while being lowered into the well. Such precautions may comprise firm anchorage of the cord to the wire line at closely spaced intervals and/or the use of cord in accordance with the present invention incorporating built-in stretch resisting means such as non-stretching small wires or other high strength non-stretching filaments 68,68. These may take the place of a corresponding number of filaments of the outer protective sheath of the cord in the manner shown in FIG. 9. As there shown, the non-stretching filaments 68 are wrapped spirally in opposite directions between adjacent spiral wrappings 69,69 of the conventional cord sheath. It will be understood that the explosive cord may be fabricated in various manners well known to those skilled in this art and typically including a central core of explosive material 70 encased within a tough flexible tubular shroud 71 of plastic or the like. The latter is embraced by the wax covered braided sheath 69,69.

Additionally or alternatively, the cord may include high strength filaments extending parallel to one another lengthwise of the cord. Typically these may comprise a multiplicity of filaments such as glass fibers or a thin metal strip. As shown in FIG. 9, a band or ribbon 68 of glass fibers flexibly bonded together and wide enough to encircle the underlying layer of cord 65 is bonded or otherwise fixed to the latter layer. As here shown, the non-stretching filaments are positioned beneath the braided sheath or wrapping 69,69 but it will be understood that ribbon 68 could be applied lengthwise of the exterior of this sheath if desired. In this event, the non-stretching ribbon can be applied to conventional cord at the time of use and restricted to those portions of the cord exposed to stretch-producing hazards.

The invention explosive cord as herein illustrated also includes a suitable heat resisting shield effective to retard the passage of heat to the explosive core proper 70. Such heat shield may comprise any flexible heat insulating material or a thin flexible layer of polished heat reflecting material indicated at 72 in FIG. 9. This shield may be wrapped spirally about tube 71 either beneath or exteriorly of ribbon 68'.

Desirably the explosive cord 65,66 is assembled lengthwise of the wire line in a taut condition, and preloaded to a suitable uniform tension. According to this invention, this is accomplished in the manner illustrated in FIG. 5 and showing the tensioning equipment for one only of the cords, it being understood that cord 66 passes over similar automatic tensioning equipment. The cord passes from a supply reel 73 mounted on a suitable support 74, and thence over an idler pulley 75. The cord is held firmly seated on pulley 75 by a pivotally supported idler pulley 76 and a tension spring 77 The cord then passes over a pair of idlers 78 between which there is mounted a pulley 79 on which any suitable member of removable tension-regulating weights 80,80 can be mounted. The cord then passes over an idler 82 carried by the hanger 83 for wire line guide pulley 30, pulley 82 being appropriately positioned close to one side of the wire line at the point of entry into the well head.

It will be understood that the wire line leaves the power-driven wire line reel carrying only mandrels 41 or their equivalent such as collars 60 (FIG. 13). As these'compone'nts approach entry into the well head,

the wire line is stopped while the crew inserts the main body 48 or 48' of the centralizer and secures it in place by tightening thumb nut 50 or 50. At the same time the explosive cords are securely anchored to the main boy of the centralizer by serving the same with tapes or tiebands 85 (FIG. l). The crew then proceeds to lower the wire line along with the attached explosive cords under appropriate tension as determined by the weights 80 carried by the pulley 79. Centralizers continue to be assembled to the wire line in succession and anchored to the explosive cords as described.

If the centralizers are spaced considerable distances apart or in cases where smaller size explosive cord is being used, it is desirable to anchor the cords to the wire line at one or more points between adjacent centralizers. This is done using cylinders 90 constructed as shown in FIGS. 7 and 8. These anchor members are secured to the wire line by swaging, brazing or other suitable means. Each member 90 is formed with long grooves 91 of V-shape in cross section extending lengthwise of the body and sized to grip cords 65,66 firmly irrespective of the particular size being used. A deep annular groove encircling the mid-portion of member 90 accommodates tape or banding 93 applied over the cords 65,66 to clamp the latter firmly within grooves 91.

The equipment employed in connection with the two conductors 26,27 of the wire line to detonate the explosive cords according to one preferred embodiment is illustrated schematically in FIG. 6. The upper ends of conductors 26,27 are connected to battery 28 by way of a double throw reversing switch 95 and a rheostat 96. The lower end of the wire line closest to the upper end of the explosive cords is connected to the cord by way of an electrical discriminator 97. This discriminator preferably comprises a thick-walled tubular housing 98 of non-conductive material charged with potting compound encapsulating at least one and preferably several semi-conductors 99a,99b, connected in series parallel with one of the wire line conductors as conductor 27. The other conductor 26 is connected to a common bus 100 connected in turn to one terminal of each of the cord detonators 101. The other lead 102 of each detonator is connected to a terminal post 103 in the side wall of discriminator 98 and connected as shown in FIG. 6 to a respective one of the semiconductors 99a,99b.

The detonators 101 are preferably arranged in vertically staggered relation as indicated in FIGS. 1 and 6 and are sufficiently spaced from one another that the detonation of one does not cause detonation of any other detonator, or the premature ignition of any explosive cord.

In some service operations only two or three explosive cords are secured to the wire line during single run in operations. In this case, ordinary diodes may be used. However when employing a larger number of string shots, such as is indicated in FIG. 6 at 65'66, 65",66, 65", then it is desirable to employ Zenertype diodes 99a,99b of an appropriate voltage rating to be triggered by a particular voltage applied thereto by adjusting the contactor of rheostat 96. It will be noted that diodes 99a are connected in series to pass current of a particular polarity whereas diodes 991) are connected in series with one another in a parallel circuit and are triggered to pass current only by voltage of an opposite polarity.

The operation of the equipment will now be described, it being assumed that perforations 11 of well casing 10 are plugged with sand or other debris interfering with the well flow. To remove this debris and restore full flow conditions, the service crew connects the blow out preventor assembly to the top of the well casing using assembly bolts 15 and then proceed to lower weight 35 of the wire line toward the well head. As this weight is about to disappear into the casing the main body assembly 48 of the first centralizer is inserted crosswise of the wire line and locked in place by tightening thumb screw 51. The selected number of string shot cords are then threaded through the tension control equipment shown in FIG. 5 and their free ends are firmly secured to the opposite sides of the first centralizer by banding 85. The wire line is then lowered into the well and successive ones of the centralizer bodies are assembled to the line and taped in place. If the intermediate cord anchorages 90, such as those shown in FIGS. 6 and 7 are employed, the explosive cords are inserted into the V-shaped slots 91 of these anchorages and taped firmly in place.

After all of the centralizers have been installed and the tensioned explosive cords have been secured to the sides of the wire line in the mannerjust described, their upper ends are cut off in echelon and each is provided with a detonator cap 101 and connected in circuit with a respective terminal 103 of discriminator 97. The lead wires to the discriminator and to the detonator bus 100 are also completed following which the wire line is lowered into the well until the string shots are located directly opposite the perforations to be cleared of debris. Cover 17 of the blow out preventor is then closed and locked in place by thumb nuts 22 to avoid any possibility that the string shot operations will open a high pressure area of the earth formation into the well and initiate high pressure flow from the top of the well.

Before closing switch 95, the operator makes certain that rheostat regulator 96 is adjusted to its upper end or a harmless position ineffective to activate any detonator. The switch is then closed and the first cord is fired by adjusting rheostat 96 downwardly until the voltage increases sufficiently to tire the first string shot 65. No one of the other string shots 66,66 or 66 will fire because the voltage is inadequate to trigger any one of the Zener diodes 99a. The second string shot 66 will be detonated simply by shifting the rheostat in a direction to increase the applied voltage to a value triggering the upper one of diodes 99a. In this same manner the operator can proceed to detonate cords 66 and 66" in any desired time interval by regulation of rheostat 96.

He may then proceed to detonate the remainder of the cords by returning the rheostat to its initial position and close switch 95 in the opposite direction thereby reversing the polarity of the applied voltage. The operator then proceeds to adjust the voltage in steps as before to values adequate to fire cords 65", 65".

Should the operator desire to fire more than one cord simultaneously, he adjusts the voltage regulator 96 to the appropriate voltage known as required to'as sure triggering the desired number of diodes and then closes the switch in the proper direction to trigger these selected diodes.

It will therefore be apparent from the foregoing that a highly versatile technique has been provided for performing string shot service operations on oil wells having faulty flow characteristics. The technique makes it possible to employ any desired number of string shots using the smallest available grain rating and including a series of larger grain rating and to explode these in succession or in any desired groupings until full flow conditions are restored. The technique permits well flow tests to be conducted between individual detonations to determine the effectiveness thereof, following which additional explosions of any selected size and number may be performed or, if tests show that full flow has been restored, the servicing operation can be immediately discontinued and the wire line withdrawn from the well.

The withdrawal operation is performed after opening cover 17 and then reeling in the line until the discriminator 97 and the. centralizer exit from the well head. These are removed as are successive other centralizers in a simple and expeditious manner until the withdrawal operation has been completed.

Although the improved and versatile explosive cord provided by this invention has been described in connection with a particular application, it will be recognized that this application is merely illustrative of many modes of utilization all having obvious and marked ad vantages over prior constructions.

While the particular method of conducting string shot operations herein shown and disclosed in detail is fully capable of attaining the objects and providing the advantages hereinbefore stated, it is to be understood that it is merely illustrative of the presently preferred embodiments of the invention, and that no limitations are intended to the details of construction or design herein shown other than as defined in the appended claims.

] claim:

1. An explosive detonation conveying cord comprising an impervious non-metallic flexible tube means charged with a rapidly combustible material, protective sheath means enclosing said tube, and high-strength non-stretchable flexible means held pressed against the exterior surface of one of said aforementioned means and extending substantially parallel to the axis of said cord effective to substantially increase the tensile strength of said cord and to resist stretching of said cord by tensile forces acting lengthwise thereof and malfunctioning of said combustible material due to stretch of said cord by such tensile forces.

2. An explosive cord as defined in claim 1 characterized in the provision of heat shield means embracing the same from end to end thereof and positioned outwardly of said flexible tube means, said heat shield being effective to retard the passage of heat therepast and toward said charge of material within said flexible tube means.

3. An explosive cord as defined in claim 1 characterized in that said non-stretchable flexible means is metallic.

4. An explosive cord as defined in claim 1 characterized in that said non-stretchable flexible means is non-metallic.

5. An explosive cord as defined in claim 1 characterized in that said non-stretchable flexible means comprises a plurality of substantially non-stretchable filaments extending lengthwise of said cord and substantially parallel to the axis thereof.

6. An explosive cord as defined in claim 5 characterized in that said substantially non-stretchable filaments include a multiplicity of glass fibers arranged in a layer embracing said flexible tube means.

7. An explosive cord as defined in claim 6 characterized in that said non-stretchable filaments are sandwiched between said flexible tube means and said protective sheath means.

8. An explosive cord as defined in claim 2 characterized in that said heat shield includes heat reflective material.

9. An explosive detonation conveying cord comprising an impervious non-metallic flexible tube charged with a rapidly combustible material, a protective sheath enclosing said tube, and a heat shield sheath of heat reflecting metallic foil enclosing said cord from end to end thereof outwardly of the exterior side of said flexible tube and effective to reflect heat outwardly from and to retard the flow of heat therepast and to said charge of combustible material enclosed by said flexible tube. 

1. An explosive detonation conveying cord comprising an impervious non-metallic flexible tube means charged with a rapidly combustible material, protective sheath means enclosing said tube, and high-strength non-stretchable flexible means held pressed against the exterior surface of one of said aforementioned means and extending substantially parallel to the axis of said cord effective to substantially increase the tensile strength of said cord and to resist stretching of said cord by tensile forces acting lengthwise thereof and malfunctioning of said combustible material due to stretch of said cord by such tensile forces.
 2. An explosive cord as defined in claim 1 characterized in the provision of heat shield means embracing the same from end to end thereof and positioned outwardly of said flexible tube means, said heat shield being effective to retard the passage of heat therepast and toward said charge of material within said flexible tube means.
 3. An explosive cord as defined in claim 1 characterized in that said non-stretchable flexible means is metallic.
 4. An explosive cord as defined in claim 1 characterized in that said non-stretchable flexible means is non-metallic.
 5. An explosive cord as defined in claim 1 characterized in that said non-stretchable flexible means comprises a plurality of substantially non-stretchable filaments extending lengthwise of said cord and substantially parallel to the axis thereof.
 6. An explosive cord as defined in claim 5 characterized in that said substantially non-stretchable filaments include a multiplicity of glass fibers arranged in a layer embracing said flexible tube means.
 7. An explosive cord as defined in claim 6 characterized in that said non-stretchable filaments are sandwiched between said flexible tube means and said protective sheath means.
 8. An explosive cord as defined in claim 2 characterized in that said heat shield includes heat reflective material.
 9. An explosive detonation conveying cord comprising an impervious non-metallic flexible tube charged with a rapidly combustible material, a protective sheath enclosing said tube, and a heat shield sheath of heat reflecting metallic foil enclosing said cord from end to end thereof outwardly of the exterior side of said flexible tube and effective to reflect heat outwardly from and to retard the flow of heat therepast and to said charge of combustible material enclosed by said flexible tube. 